1. Field of the Invention
The present invention relates to a method and an apparatus for fabrication of a specimen. More particularly, the present invention relates to a method and an apparatus for extracting a micro-specimen including a specific small area of a semiconductor material such as a semiconductor wafer or a semiconductor device chip from the semiconductor material by separation using an ion beam and for fabricating a specimen used for carrying out an observation, an analysis and/or a measurement of the specific small area.
2. Description of the Prior Art
In recent years, efforts made to shrink geometries of semiconductor devices make progress at a very great pace. In a structure analysis of these semiconductor devices, there has been demanded an observation of a nanoscopic structure which is so small that, at a resolution of an ordinary scanning electron microscope referred to hereafter simply as an SEM, the structure can not be observed any longer. As a result, observation by means of a transmission electron microscope which is abbreviated hereafter to a TEM is indispensable in place of an SEM. Traditionally, however, fabrication of a specimen for an observation using a TEM can not help resorting to manual work which must be done by a well trained person and takes a long time. For this reason, in reality, the method for observation of a specimen using a TEM does not come into wide use as the method for observation by means of an SEM, whereby a specimen can be fabricated with ease and results of observations can be thus be obtained immediately, did.
The conventional method for fabrication of a specimen for an observation by using a TEM is explained as follows. FIG. 2 is diagrams showing the first conventional method for fabrication of a specimen for observation using a TEM. A specimen for observation using a TEM is also referred to hereafter simply as a TEM specimen. To be more specific, FIG. 2/(a) is a diagram showing a semiconductor wafer 2 on which LSIs were fabricated. The semiconductor wafer 2 is referred to hereafter simply as a wafer or a substrate. As shown in FIG. 2/(b), the wafer 2 comprises an upper-layer portion 2A and a lower-portion 2B or a substrate. Assume that a specimen for TEM observation of a specific area on the wafer 2 is fabricated. First of all, a mark not shown in the figure is put on an area 22 subjected to the observation using a TEM. By exercising care so as not to damage the area 22 to be observed, an injury is deliberately inflicted on the wafer 2 by using a tool such as a diamond pen in order to cleave the wafer 2 or the wafer 2 is cut by means of a dicing saw in order to take out a sliber chip 21 shown in FIG. 2/(b). In order to make the center of a TEM specimen being created the area 22 to be observed, the areas 22 of two chips are stuck to each other by using adhesive 23 to produce 2 specimens 24 stuck together as shown in FIG. 2/(c). Then, the two stuck specimens 24 are sliced by means of a diamond cutter to produce slice specimens 25 shown in FIG. 2/(d). The dimensions of each of the slice specimens 25 are about 3 mmxc3x973 mmxc3x970.5 mm. Then, the slice specimen 25 is put on a grinding plate to be ground by using abrasives into a thin specimen, namely, a ground specimen 25xe2x80x2 with a thickness of about 20 microns. Subsequently, the ground specimen 25xe2x80x2 is attached to a single-hole holder 28 mounted on a TEM stage, that is, a stage for holding a TEM specimen as shown in FIG. 2/(e). Then, ion beams 27 are irradiated to the surfaces of the ground specimen 25xe2x80x2 as shown in FIG. 2/(f). Sputtering fabrication (or ion-milling fabrication) is then carried out on the center of the specimen 25xe2x80x2 as shown in FIG. 2/(g). Finally, when a hole has been bored through the center of the specimen 25xe2x80x2, the irradiation of the ion beams 27 is halted as shown in FIG. 2/(h). A thinned area 26 with a thickness not exceeding a value of about 100 nm fabricated as described above has been observed by a TEM. This method is described in references such as a book with a title of xe2x80x9cHigh-Resolution Electron Microscope: Principle and Usagexe2x80x9d, authored by Hisao Horiuchi and published by Kyoritsu Syuppan, Page 182, and used as prior-art reference 1.
FIG. 3 is a diagram showing the second conventional method for fabrication of a TEM specimen. This method is a method for fabrication of a specimen using a focused ion beam which is abbreviated hereafter to an FIB. As shown in the figure, first of all, a mark not shown in the figure is created by using a laser beam or an FIB in the vicinity of an area 22 to be observed on the wafer 2 and then the wafer 2 is diced as shown in FIG. 3/(a). A sliver chip 21 shown in FIG. 3/(b) is then taken out from the wafer 2. The sliver chip 21 is further sliced to produce slice specimens 21xe2x80x2 shown in FIG. 3/(c). The dimensions of each of the slice specimens 21xe2x80x2 are about 3 mmxc3x970.1 mmxc3x970.5 mm which is the thickness of the wafer 2. Then, the slice chip 21xe2x80x2 is ground into a thinned specimen 21xe2x80x3. The thinned specimen 21xe2x80x3 is then stuck to a TEM-specimen holder 31 which resembles a thin metallic disc plate and has a cut portion 31xe2x80x2 as shown in FIG. 3/(d). Subsequently, the area 22 to be observed on the thinned specimen 21xe2x80x3 is further thinned by means of an FIB 32 so that only a slice 22xe2x80x2 having a thickness of about 100 nm is left as shown in FIG. 3/(e), (f). The slice 22xe2x80x2 is used as a specimen for an observation using a TEM. This method is described in documents such as a collection of theses with a title of xe2x80x9cMicroscopy of Semiconducting Materials 1989xe2x80x9d, Institute of Physics Series No. 100, Pages 501 to 506, which is used as prior-art reference 2.
FIG. 4 is a diagram showing the third conventional method for fabrication of a TEM specimen. The method is disclosed in Japanese Patent Laid-open No. Hei 5-52721 which is used as prior-art reference 3. As shown in the figure, first of all, a specimen substrate 2 is held in such a posture that an FIB 32 is irradiated to the surface of the specimen substrate 2 perpendicularly. The surface of the specimen substrate 2 is then scanned by the FIB 32 along the circumference of a rectangle to form a rectangular hole 33 with a sufficient thickness on the surface as shown in FIG. 4/(a). Then, the specimen substrate 2 is inclined so that the surface thereof forms a gradient of about 70 degrees with the axis of the FIB 32 and a bottom trench 34 for separation is further created on a side wall of the rectangular hole 33 as shown in FIG. 4/(b). The gradient angle of the specimen substrate 2 is adjusted by using a sample stage which is not shown in the figure. Subsequently, the orientation of the specimen substrate 2 is restored to its original posture so that the FIB 32 is again irradiated to the surface of the specimen substrate 2 perpendicularly and a trench 35 is further created as shown in FIG. 4/(c). Then, by driving a manipulator for holding a probe 36, the tip of the probe 36 is brought into contact with the surface of a portion 40 of the specimen substrate 2 to be separated as shown in FIG. 4/(d). It should be noted that the manipulator itself is not shown in the figure. In this state, the FIB 32 is irradiated to a local area including the tip of the probe 36 while gas 39 for deposition is being supplied from a gas nozzle 37 to create an ion-beam-assisted-deposition film 38 which is abbreviated hereafter to an IBAD film or a deposition film. In this way, the portion 40 of the specimen substrate 2 to be separated and the tip of the probe 36 which have been brought into contact with each other are firmly joined to each other by the deposition film 38 as shown in FIG. 4/(e). Finally, portions left around the portion 40 of the specimen substrate 2 to be separated are separated by the FIB 32 to detach the portion 40 from the specimen substrate 2 as shown in FIG. 4/(f). The detached portion 40 separated from the specimen substrate 2 remains in a state of being firmly joined to the tip of the probe 36 as shown in FIG. 4/(g). An area on the separated portion 40 to be observed is further thinned by using an FIB to a thickness of about 100 nm to produce a specimen for observation using a TEM.
The first and second conventional methods described above can not help resorting to manual work requiring skills of a well trained person fabricating the specimen. The manual work includes grinding, mechanical fabrication and sticking the specimen to the TEM-specimen holder. In addition, with these conventional methods, in order to fabricate a desired specimen, it is necessary to split the wafer or the substrate of the device chips into portions by cleaving or cutting the wafer or the substrate. In order to acquire a specimen of a desired area, portions adjacent to the desired area are inevitably and/or inadvertently cleaved or cut. Assume that it is necessary to observe and/or analyze a portion other than an area which was subjected to an observation and/or an analysis before. Since the substrate of the specimen was once cut in order to fabricate specimens for the prior observation and/or analysis, an injury and/or a damage was inevitably and/or inadvertently inflicted upon the portion subjected to the next observation and/or analysis or a positional relation among portions to be observed and/or analyzed is no longer known. As a result, there is raised a problem that accurate information on observations and/or analyses can not be obtained continuously due to the inflicted injury and/or damage. In addition, while the ion milling and the process to thin a film by using an FIB described above do not directly involve manual work, they have a problem of a long fabrication time which is difficult to solve.
Furthermore, in recent years, there is seen a trend of an increasing wafer diameter to 300 mm. The number of device chips that can be fabricated from such a wafer also increase as well. In addition, the device itself has more added values. As a result, splitting a wafer into portions by cleaving or cutting the wafer in order to observe and/or analyze a particular area leads to a disposal to discard portions other the area to be observed and/or analyzed which is very uneconomical. Moreover, when a small particle or an abnormal shape is detected in a certain area during a scanning operation over the entire wafer by driving a variety of microscopes, a cause of such a small particle or such an abnormal shape has to be clarified by conducting an observation and/or an analysis prior to the splitting a wafer into chips, in particular, before the small particle disappears. Otherwise, a number of defective devices among final products will be resulted in, incurring an even larger loss. If a plurality of specimens can be produced in a short period of time without splitting the wafer into portions, observations and/or analyses can be carried out very economically, giving rise to a great contribution to improvements of a product manufacturing yield.
With the third conventional method, on the other hand, once a specimen is set on the sample stage, it is not necessary for the operator to do manual work directly till separation of micro-specimens and to cut the wafer carelessly. In this method, however, the separated specimen remains in a state of being attached to the tip of a probe so that, when the separated specimen is brought into an observation apparatus and/or an analyzer in such a state to be observed and/or analyzed, the specimen will vibrate, raising a problem that it is impossible to obtain reliable results of observation and/or analysis.
As the conventional TEM-specimen holder, a holder 78 with a single hole 79 shown in FIG. 7/(a), a holder 80 with a notch 108 shown in FIG. 7/(b) and a holder 109 with a mesh shown in FIG. 7/(c) are known. Assume that the single-hole-type holder 78 or the notch-type holder 80 is used in the third conventional method for specimen fabrication described above to hold a micro-specimen 40 with a small size in the range 20 to 30 microns. In this case, it is necessary to adjust the position of the micro-specimen 40 on the inner wall of the notch 108 or the single hole 79 with a high degree of accuracy, making the installation work difficult to carry out. Such a problem is not encountered with the mesh-type holder 109. This is because, by using a mesh-type holder 109 with a gap between mesh nodes adjusted to the size of the micro-specimen 40, the position at which the micro-specimen 40 is to be installed can be selected arbitrarily to a certain degree. With the mesh-type holder 109, however, an electron beam path 82 propagating toward an area 81 to be observed is shielded by a mesh structure member 109xe2x80x2 as shown in FIG. 7/(d), making an observation using a TEM impossible in some cases.
It is thus an object of the present invention to provide an improved method for fabrication of a specimen capable of solving the problems encountered in the conventional methods described above and to provide a good apparatus for fabrication of a specimen used for implementing the improved specimen fabrication method.
To be more specific, it is a first object of the present invention to provide a specimen fabrication method capable of fabricating a specimen of a small area to undergo an observation or a measurement/analysis carried out by an observation apparatus such as a TEM or a measurement/analysis apparatus to which the specimen is to be transferred without the need for a well trained person to do manual work such as grinding and dicing and the need to split a semiconductor wafer or an LSI chip by cleaving or cutting.
It is a second object of the present invention to provide a good specimen fabrication apparatus used for implementing the specimen fabrication method provided as the first object of the invention.
It is a third object of the present invention to provide a TEM-specimen holder which is used in conjunction with a TEM and allows a micro-specimen extracted from a specimen substrate to be,positioned with ease.
In order to achieve the first object of the present invention described above, the present invention provides a specimen fabrication method which comprises the steps of:
firmly joining the tip of a probe to the vicinity of an area on a specimen substrate such as an LSI chip and a semiconductor wafer held on a sample stage to be subjected to a desired observation and/or a measurement/analysis; (such an area is also referred to hereafter as an area to be observed)
irradiating an ion beam to regions surrounding the vicinity of the area to be observed;
extracting and separating a micro-specimen including the area to be observed from the specimen substrate by ion-beam sputtering fabrication;
conveying the extracted and separated micro-specimen with the micro-specimen firmly joined to the tip of the probe as it is to a TEM-specimen holder of an apparatus for conducting the desired observation and/or measurement/analysis by moving the probe or the sample stage;
firmly attaching the micro-specimen to the TEM-specimen holder;
separating the tip of the probe from the micro-specimen; and
carrying out the desired observation and/or measurement/analysis which is also generically referred to hereafter simply as an observation.
In addition, in order to carry out the observation on a specific area to be observed on the specimen substrate, before firmly joining the tip of the probe to the vicinity of the specific area to be observed, a marking process of putting a mark on the specific area is performed in order to clearly indicate the specific area. After the micro-specimen has been separated from the tip of the probe, an FIB is irradiated to the specific area to be observed as indicated by the mark in order to carry out additional fabrication such as film thinning.
It should be noted that in the process of firmly joining the tip of the probe to the vicinity of the specific area to be observed, the tip can be joined to the vicinity through an ion-beam assist deposition film or a redeposition film created by ion-beam sputtering or joined by a fusion or metallic-junction technique.
In the process of separating the tip of the probe from the micro-specimen, on t,he other hand, an ion-beam sputtering fabrication method can be adopted. As an alternative, if a method of using adhesive as a technique of firmly joining the tip of the probe to the micro-specimen, in the process of separating the tip of the probe from the micro-specimen, an UV-ray irradiation method or a heating method can be adopted. As another alternative, a method of electrostatic absorption can be adopted as a technique of firmly joining the tip of the probe to the micro-specimen.
In addition, in order to achieve the second object of the present invention described above, the present invention provides a specimen fabrication apparatus which comprises:
a movable sample stage on which a specimen substrate is mounted;
a probe connecting means for joining the tip of a probe to the vicinity of a desired area to be observed on the specimen substrate;
a micro-specimen separating means for separating a micro-specimen including the area to be observed from the specimen substrate with the micro-specimen joined to the tip of the probe as it is by irradiation of an ion beam to regions surrounding the vicinity of the area to be observed;
a micro-specimen fixing means for firmly fixing the micro-specimen separated from the specimen substrate to a TEM-specimen holder; and
a probe separating means for separating the tip of the probe from the micro-specimen firmly fixed to the TEM-specimen holder.
The sample stage comprises a sample cassette and a movable sample cassette holder for holding the sample cassette. The sample cassette is used for holding the TEM-specimen holder or a cartridge of the TEM-specimen holder which can be mounted and removed on and from the sample stage of the observation apparatus.
Typically, a probe exhibiting a spring effect can be used as the probe described above.
The probe connecting means typically comprises a probe contact means for bringing the tip of the probe into contact with the surface of the specimen substrate, and a deposition-film forming means for forming an ion-beam assist deposition film (an IBAD film) at the contact portion between the tip of the probe and the surface of the specimen substrate. Typically, the probe contact means has a manipulator mechanism for holding the probe and moving the probe relatively to the surface of the specimen substrate. On the other hand, the deposition-film forming means typically comprises an ion-beam irradiating optical system for irradiating an ion beam to the contact portion between the tip of the probe and the surface of the specimen substrate, and a gas supplying means for supplying gas for assisted deposition to the contact portion to which the ion beam is irradiated. The tip of the probe is firmly joined to the surface of the specimen substrate through the IBAD film formed by the deposition-film forming means.
The micro-specimen separating means has a configuration including an ion-beam irradiating optical system for irradiating an ion beam to the specimen substrate. The ion-beam irradiating optical system is typically a PJIB (projection ion beam) irradiating optical system comprising an ion source and a projection optical system for projecting ions emitted from the ion source on the specimen substrate as a PJIB. As an alternative, the ion-beam irradiating optical system can be an FIB (focused ion beam) irradiating optical system comprising an ion source and a focusing optical system for irradiating ions emitted from the ion source on the specimen substrate as an FIB. As another alternative, the ion-beam irradiating optical system can be a combination of the PJIB irradiating optical system and the FIB irradiating optical system. By irradiation of an ion beam which can be a PJIB or an FIB to the specimen substrate by means of the ion-beam irradiating optical system, the specimen substrate is subjected to sputter fabrication allowing the micro-specimen to be extracted and separated from the specimen surface. In addition, the micro-specimen separating means can also be configured to include a first ion-beam irradiating optical system for irradiating an ion beam to the specimen substrate from a first direction and a second ion-beam irradiating optical system for irradiating an ion beam to the specimen substrate from a second direction different from the first direction. By providing the two ion-beam irradiating optical systems in this way, the process to extract a micro-specimen from the specimen substrate can be carried out more easily. It should be noted that, as the micro-specimen separating means, a laser-beam irradiating optical system or a combination of an ion-beam irradiating optical system and a laser-beam irradiating optical system can also be used as well.
Typically, the micro-specimen fixing means comprises a specimen contact means for bringing a micro-specimen into contact with an area on the TEM-specimen holder to fix the micro-specimen to the area and a deposition-film forming means for forming an ion-beam assist deposition film (an IBAD film) at the contact portion between the micro-specimen and the area on the TEM-specimen holder to fix the micro-specimen to the area. The deposition-film forming means can have the same configuration as the deposition-film forming means employed in the probe contact means described earlier. The micro-specimen is firmly joined to the area on the TEM-specimen holder to fix the micro-specimen to the area through the IBAD film formed by the deposition-film forming means.
The probe separating means is implemented typically by a means for irradiating an ion beam to the IBAD film through which the micro-,specimen is firmly joined to the area on the TEM-specimen holder. By irradiation of an ion beam, the IBAD film fixing the tip of the probe to the micro-specimen is subjected to a sputtering process to remove the IBAD film, hence, allowing the tip of the probe to be pulled out from the micro-specimen.
It should be noted that the probe connecting means and the micro-specimen fixing means can also use a redeposition film formed by ion-beam sputtering in place of an IBAD film or adopt a fusion or metallic-junction method. In this case, the probe separating means adopts the ion-beam sputtering fabrication. In addition, the probe connecting means and the micro-specimen fixing means can also adopt an adhesion method or an electrostatic absorption method instead of the methods described above.
The specimen fabrication apparatus provided by the present invention may include an observation unit for observing the surface of the specimen substrate, the tip of the probe or the vicinity of the TEM-specimen holder. The observation unit typically comprises an electron-beam irradiating optical system for irradiating an electron beam to the aforementioned member to be observed, a secondary-electron detector for detecting secondary electrons emitted by the observed member due irradiation of the electron beam and a display sub-unit for displaying a secondary-electron image of the observed member by using a detection signal output by the secondary-electron detector. As an alternative, the observation unit can also be implemented by an optical observation apparatus such as an optical microscope. By observing the member to be observed using the observation unit, it is possible to obtain information on a contact/connection state between the tip of the probe and the surface of the specimen substrate, a separation state of the micro-specimen from the surface of the specimen substrate and a contact/connection state between the micro-specimen and the TEM-specimen holder.
In addition, the specimen fabrication apparatus provided by the present invention may also be provided with a detector for detecting a contact/connection state as well as a separation state between the tip of the probe and the surface of the specimen substrate, between the micro-specimen and the specimen substrate and between the micro-specimen and the TEM-specimen holder. The detector can make use of variations in contact resistance between the members brought into contact with each other or variations in voltage contrast on the secondary-electron image mentioned above. By virtue of the detector, it is possible to obtain information on the contact/connection state and the separation state between the respective members with a high degree of accuracy.
The TEM-specimen holder typically comprises a metallic wire for holding the micro-specimen and a support unit for firmly supporting both the ends of the metallic wire. In the configuration of the TEM-specimen holder, the micro-specimen is firmly held by the metallic wire, allowing a specimen holding system suitable for observation using a TEM to be realized.
Other objects of the present invention, its configurations and effects provided thereby will become apparent one after another from the following detailed description of embodiments.